Invert Dictionary mappings in C#

This post will discuss how to invert Dictionary mappings in C#.

1. Using `foreach` loop

The idea is to create a new instance of Dictionary<TValue,TKey> for a given dictionary of type Dictionary<TKey,TValue>. Then use a foreach loop to iterate over the dictionary, and insert each mapping into the new Dictionary in reverse order of its key-value pair. The following code demonstrates this.

using System;

using System.Collections.Generic;

public static class Extensions

{

public static Dictionary<V, K> Reverse<K, V>(this IDictionary<K, V> dict)

{

var inverseDict = new Dictionary<V, K>();

foreach (var kvp in dict)

{

if (!inverseDict.ContainsKey(kvp.Value)) {

inverseDict.Add(kvp.Value, kvp.Key);

}

return inverseDict;

}

public class Example

{

public static void Main()

{

Dictionary<string, int> dict = new Dictionary<string, int>() {

{"A", 1}, {"B", 2}, {"C", 3}

};

var inverseDict = dict.Reverse();

Console.WriteLine(String.Join(", ", inverseDict)); // [1, A], [2, B], [3, C]

}

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Note that for any repeated values in the given dictionary, the above solution considers the first encountered value and ignores all other values.

2. Using `Dictionary<TKey,TValue>.ToDictionary()` Method

Another option is to use the Dictionary<TKey,TValue>.ToDictionary() method to accumulate the key-value pairs into a new dictionary in reverse order. Note that this assumes that the reverse mapping is well-defined, and throws System.ArgumentException if the values in the original map are not unique.